A piezo actuator utilizes the piezoelectric effect of PZT or another piezoelectric material and has a piezo stack, which is a capacitive element. When the piezo stack is electrically charged or discharged, it expands or contracts, linearly moving a piston or the like. For example, in fuel injection systems for internal combustion engines, piezo actuators switch ON-OFF valves of fuel injectors.
A piezo actuator has a temperature characteristic, in which the capacitance of its piezo stack changes with temperature. The amount of displacement of the piezo actuator based on the expansion, contraction or displacement of the piezo stack also changes with temperature. It is known that this temperature characteristic is compensated if the energy supplied to the piezo stack by a piezo actuator drive circuit is kept constant.
JP-A-2002-136156 (U.S. Pat. No. 6,230,190) discloses a piezo actuator drive circuit that makes use of this to drive a piezo actuator. In this publication, it is proposed to control a piezo stack with constant energy. This method was proposed as a driving method for compensating for the temperature characteristic of the amount of displacement of a piezo actuator. FIG. 10 shows the circuit for driving the piezo actuator for one cylinder of an internal combustion engine by this proposed method. FIGS. 11A, 11B and 11C show the charging control for the piezo stack in the drive circuit comparatively in a case where the capacitance of the piezo stack varies due to its temperature characteristic. In FIGS. 11A, 11B and 11C, the capacitances C of the piezo stack are 6, 8 and 10 microfarads (μF), respectively.
With reference to FIG. 10, the piezo actuator drive circuit includes a first current-carrying path and a second current-carrying path. The first current-carrying path carries electric current from a DC power supply 11 through a switching device 14 and an inductor 16 to a piezo stack 7. The second current-carrying path bypasses the power supply 11 and the switching device 14 and carries electric current through the inductor 16 to the piezo stack 7. While the switching device 14 is ON, a gradually increasing charging current flows through the first current-carrying path. While the switching device 14 is OFF, a gradually decreasing charging current flows through the second current-carrying path due to the flywheel effect. If the switching device 14 is repetitively turned on and off, the charging current repeats a gradual increase and decrease, as shown in FIG. 11A. This increases the amount of charge on the piezo stack 7, so that the voltage Vp across the piezo stack 7 rises step by step.
Specifically, it is necessary in advance to find the ON periods for which the switching device 14 is ON so that the piezo stack 7 can be charged with a desired amount of energy E when its capacitance C is the central value of 8 μF. While the piezo stack 7 is charged, its capacitance C may increase from the central value of 8 μF (FIG. 11B) to 10 μF (FIG. 11C). The capacitance increase suppresses the speed at which the voltage across the piezo stack 7 rises. The speed suppression suppresses the speed at which the voltage applied to the inductor 16 drops.
Accordingly, the speed at which the charging current I decreases on the whole is suppressed in comparison with that for the central value of 8 μF. This increases the speed at which the piezo stack 7 is charged. Consequently, even if the capacitance of the piezo stack 7 increases, the speed at which the voltage Vp across it rises is not greatly suppressed. Even if this capacitance C increases, the speed at which the charging current decreases on the whole is not greatly suppressed.
The suppression of the speed at which the voltage across the piezo stack 7 rises decreases the speed at which the stack 7 is supplied with energy. The suppression of the speed at which the charging current I decreases raises the speed at which the piezo stack 7 is supplied with energy. Consequently, these cancel each other, so that the time profile of the supplied energy is roughly constant regardless of the increase of the capacitance of the piezo stack 7 (FIGS. 11B and 11C).
The capacitance C of the piezo stack 7 may decrease from the central value of 8 μF (FIG. 11B) to 6 μF (FIG. 11A). Because the ON periods, when the switching device 14 is ON, are constant, the capacitance decrease increases the speed at which the voltage Vp across the piezo stack 7 rises. The speed increase makes higher the speed at which the voltage applied to the inductor 16 drops. Accordingly, the speed at which the charging current decreases on the whole is higher in comparison with that for the central value of 8 μF. This decreases the speed at which the piezo stack 7 is charged. Consequently, even if the capacitance of the piezo stack 7 decreases, the speed at which the voltage across it rises is not very high. Even if this capacitance decreases, the speed at which the charging current decreases on the whole is not very high either. The increase of the speed at which the voltage across the piezo stack 7 rises increases the speed at which the piezo stack is supplied with energy. The increase of the speed at which the charging current decreases lowers the speed at which the piezo stack 7 is supplied with energy. Consequently, these cancel each other, so that the time profile of the supplied energy is roughly constant regardless of the decrease of the capacitance of the piezo stack 7 (FIGS. 11A and 11B).
In this way, by making constant the time when the switching device 14 is ON, it is possible to suitably control the energy supplied to the piezo stack 7, by means of an open loop without controlling the charging of the piezo stack by detecting the variation of its capacitance.
However, piezo stacks vary in capacitance, and each of them changes in capacitance with temperature. Each piezo stack tends to change differently in capacitance with temperature. Therefore, a representative capacitance, which is roughly the central value, is sought and determined statistically in advance in consideration of the individual differences and the actually operating or working temperature. The representative capacitance is the basis for determining (storing in an ECU) the ON periods when the switching device 14 is ON so that the piezo stack 7 can be charged with the desired amount of energy. The ON periods are kept constant uniformly regardless of the temperature and the individual differences. Open control is carried out to charge the piezo stack 7 with the desired amount of energy.
For this reason, it is difficult to completely remove the error of constant energy supply quantity due to the variation among piezo stacks 7. For example, in view of the request for exhaust gas cleanup or purification in recent years, it is important that the valve switching timing of fuel injectors coincide, and it has been demanded to improve the precision with which each piezo stack is charged with a predetermined amount of energy within a predetermined time. In the conventional art, the open loop control based on the predetermined ON periods is adopted. Accordingly, it has been difficult to obtain a constant amount of displacement if the amounts of energy with which the piezo stacks are charged for the constant amount of displacement vary due to the variation among the piezo stacks.